1. Field of the Invention
The present invention relates to a non-luminous flat display device, and more particularly, to an optical sheet of a backlight unit in a liquid crystal display device.
2. Discussion of the Related Art
Recently, a variety of flat panel displays have been developed to overcome weight and volume drawbacks of cathode ray tubes (CRTs).
Such flat panel displays include a liquid crystal display (LCD) device, a field emission display, a plasma display panel (PDP), an electro-luminescence (EL) and the like. Research for enhancing the display quality and increasing the screen size of the flat panel displays are being actively performed.
Among such flat panel displays, the LCD itself is a non-luminous device, which displays images using a light source such as a lamp and has advantages of small size, lightweight, low power consumption characteristics. In particular, the LCD displays information using electrical and optical properties of liquid crystal molecules interposed in an inside of a liquid crystal display.
Unlike the CRT, the LCD essentially needs a separate unit for irradiating light, i.e., a backlight unit, because the liquid crystal molecules interposed between a thin film transistor (TFT) substrate and a color filter substrate do not emit light on their own.
The backlight unit includes: a mold frame having a receiving space; a reflector mounted on the lowest surface of the receiving space for reflecting light toward a liquid crystal display panel; a light guide plate disposed on the reflector for guiding light; a lamp unit disposed between the light guide plate and a sidewall of the receiving space for emitting light; optical sheets stacked on the light guide plate for diffusing and concentrating light; and a top chassis disposed on the mold frame for enclosing the liquid crystal display from a predetermined portion of the edge of the liquid crystal display to a side portion of the mold frame.
The optical sheets include: a diffusion sheet for diffusing light; a prism sheet disposed on the diffusion sheet for concentrating the diffused light and transferring the concentrated light to the liquid crystal display; and a protective sheet for protecting the diffusion sheet and the prism sheet.
FIG. 1 is a cross-sectional view of a related art LCD.
Referring to FIG. 1, the related art LCD 60 includes: a backlight unit 50 for generating light; and a display unit 40 for receiving the light from the backlight unit 50 and displaying images.
The backlight unit 50 includes: a lamp unit 51 for generating light; and a light guide unit for guiding the light generated by the lamp unit 51 toward the liquid crystal display 10.
The display unit 40 includes: the liquid crystal display panel 10; and an upper polarizer 30 and a lower polarizer 20 respectively disposed on an upper surface and a lower surface of the liquid crystal display panel 10. The liquid crystal display panel 10 includes: a TFT substrate 11; a color filter substrate 12 each having electrodes formed thereon; and a liquid crystal layer interposed between the TFT substrate 11 and the color filter substrate 12.
More specifically, the lamp unit 51 includes: a lamp 51a for generating light; and a lamp reflector 51b enclosing the lamp 51a. The light generated by the lamp 51a is provided into a light guide plate 52, which will be described later. The lamp reflector 51b reflects the light generated by the lamp 51a toward the light guide plate 52, thereby increasing amount of light provided to the light guide plate 52.
The light guide unit includes a reflective plate 54, the light guide plate 52, and optical sheets 53. The light guide plate 52 is disposed at one end of the lamp unit 51 to guide the light emitted from the lamp unit 51 so that the light emitted from the lamp unit 51 is incident into the LCD panel 10.
The reflective plate 54 is disposed beneath the light guide plate 52 and reflects light leaked from the light guide plate 52 toward the light guide plate 52 again.
The plurality of optical sheets 53 are disposed on the light guide plate 52 to enhance efficiency of the light passing through the light guide plate 52. Specifically, the optical sheets 53 include a diffusion sheet 53a, a prism sheet 53b, and a protective sheet 53c, and are sequentially stacked on the light guide plate 52.
The diffusion sheet 53a scatters the light that is incident from the light guide plate 52, thereby making the brightness distribution of the light uniform. The prism sheet 53b has a plurality of prisms repeatedly formed on its upper surface and concentrates the light diffused by the diffusion sheet 53a in a direction perpendicular to a plane of the LCD panel 10. Accordingly, the light that has passed through the prism sheet 53b mostly progresses in a direction perpendicular to the plane of the LCD panel 10, so that a uniform brightness distribution is obtained.
The protective sheet 53c provided on the prism sheet 53b functions not only to protect the upper surface of the prism sheet 53b, but also to diffuse light so as to make uniform the distribution of the light that is incident from the prism sheet 53b. 
FIGS. 2A and 2B are a cross-sectional view and a perspective view of the prism sheet shown in FIG. 1, respectively.
Referring to FIGS. 2A and 2B, the related art prism sheet 200 includes: a body part 210 through which the light diffused by the light guide plate and the diffusion sheet is initially introduced, and a protrusion part 220 shaped in a isosceles triangle prism having a predetermined vertical angle α, for maintaining a path of the diffused light constantly. The protrusion part 220 has a plurality of isosceles triangle prisms linearly arranged on the body part 210 in a stripe form.
In case the protrusion part 220 of a triangle prism shape is installed toward a front side, namely, the protrusion part becomes a light outputting surface and faces the liquid crystal display panel, diffused light introduced from the body part 210 is refracted and concentrated to a front side, but light incident into an incline plane of the protrusion part is lost, not contributing to improvement of a front brightness due to a total internal refraction.
To overcome such a problem, the prism sheet may be conversely arranged so that the protrusion part 220 becomes a light receiving surface and faces the light guide plate. In this case, however, a front brightness can be improved but the viewing angle becomes narrow, which is not suitable for a flat display requiring a wide viewing angle such as a television for a home use.
Further, two prism sheets may be overlapped and used in such a way that arrangement directions of the protrusion parts thereof are crossed each other. However, such an arrangement increases a number of parts used in the LCD and manufacturing costs.